Although most LEDs (light emitting diodes) today are devices which run at low voltage, the technology exists to run LEDs directly from a high voltage source, such as an AC (alternating current) line. Two common circuits for running LEDs directly from a high voltage source both place enough LEDs in a series string that their forward voltage at a desired current is approximately equal to that of the AC line voltage. In one circuit technique, two anti-parallel strings are placed directly across the AC line, with one string conducting during the first half line-cycle and the other during the second half. In a second circuit technique, a single string of LEDs is placed after a bridge rectifier, so that the single string of LEDs conducts during both halves of the line-cycle.
In both of these circuit techniques up until now, white LEDs have been created by using a string of white LED dice. It can be appreciated that this works well, if the dice are available to produce the exact shade of white, which is desired. However, in many cases the particular desired white is not available, in which case there is no way to use an AC LED, and designers must go to DC (direct current) LEDs and use complicated and expensive AC/DC converters to run them.
The other method of obtaining white light from LEDs is to use a combination of two or more different color LEDs. This is commonly done in DC LEDs. However, in circuits to date this has been impracticable in AC LEDs, because each LED color must be run at its own current. The reason for this is that to create white light requires a different amount of each LED color. Further, each LED color has different efficacy, the two factors together resulting in each LED color typically being run on its own converter.
Accordingly, it would be desirable to have the ability to select the precise shade of white delivered by an AC LED, which would permit more inexpensive LED lights to be produced for more applications. In addition, it would be desirable to generate arbitrary colored light using AC LEDs rather than needing one or more AC/DC converters.